Traditional approaches for producing lactams, used in the production of nylon, include an oxime undergoing a Beckmann rearrangement in the presence of an acid catalyst, such as fuming sulfuric acid. Exemplary reactions are shown in FIG. 1. As illustrated in FIG. 1A, cyclohexanone oxime is reacted to form ε-caprolactam. ε-caprolactam in turn is polymerized to form Nylon-6. As illustrated in FIG. 1B, cyclododecanone oxime is reacted to form ω-laurolactam. ω-laurolactam in turn is polymerized to form Nylon-12. Nylon-6 and nylon-12 are extensively used in industry and manufacturing.
One potential reaction mechanism for the reaction of FIG. 1A is illustrated in FIG. 1C. The mechanism generally consists of protonating the hydroxyl group, performing an alkyl migration while expelling the hydroxyl to form a nitirilium ion, followed by hydrolysis, tautomerization, and deprotonation to form the lactam.
Typically, Beckmann rearrangement reactions of oximes to form lactams are performed using acids such as fuming sulfuric acid. These reactions are characterized by complete or nearly complete conversion of the oxime and very high selectivity for the desired lactams. However, these reactions also produce byproducts including ammonium sulfate. Although ammonium sulfate is a useful product in itself, minimizing its production may be desirable.
Gas-phase and liquid-phase Beckmann rearrangements of cyclohexanone oximes are known, which employ various natural and synthetic catalysts including solid-acid catalysts. However, the reported results provide low conversion of the oxime and low selectivity of the desired lactam products.
Improvements in the foregoing processes are desired.